In this paper mode instability in a 500 W ytterbium-doped fiber laser is experimentally examined by changing the pumping wavelength, spectral bandwidth of signal light, active fiber temperature and coiling radius. The magnitude of power transfer from the fundamental mode to the higher order mode due to mode instability is measured as a criterion for its incident. The experiments show that the coiling radius of the first few tens of centimeters of the active fiber plays a significant role in controlling mode instability, and shifting the pumping wavelength from 976 to 973 nm can mitigate mode instability.
Modal instability (MI) and stimulated Raman scattering (SRS) are the main obstacles in the power scaling of fiber lasers and amplifiers. In the power scaling of a high-power ytterbium (Yb)-doped master oscillator power amplifier system, a new type of MI has occurred. Experimentally, it is shown that just at the onset of the SRS effect, MI takes place, and the degradation of the beam quality is observed. By the spectra and beam quality measurements, it is revealed that this type of MI can be mitigated firmly by suppressing the SRS effect in high-power Yb-doped fiber amplifiers.
To achieve a 3.02 kW Yb-doped fiber laser oscillator, the behavior of transverse mode instability (TMI) is experimentally studied in different pumping configurations; co, hybrid, counter, and bidirectional. A comparative analysis showed that population inversion saturation has a substantial impact on TMI threshold enhancement in high power fiber oscillators. Monitoring the dynamic power exchange of fundamental mode and higher-order mode of laser output beam indicates that in a hybrid pumping scheme, simultaneous pumping with two different wavelengths enhances the TMI threshold to a great stand. Moreover, injecting a few watts of pumping light in the counter direction mitigates the TMI caused by pumping in the co-direction. Calculation of population inversion in different pumping configurations using simulation shows that higher population inversion saturation leads to increasing the TMI threshold.
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